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Workers have been concerned that exposure to the chemicals could lead to serious health issues longer term.

The workers who were evaluated by an on-site medical provider were cleared Thursday to return to work.

The two workers evaluated at the hospital were examined and then released without being admitted, but still need to be evaluated by the

Hanford occupational medical provider before they can return to work.

The first incident Thursday was about 9:20 a.m. when a worker who entered the AP Tank Farm, a group of underground double-shell tanks, reported smelling vapors and developed symptoms, according to Washington River Protection Solutions.

Three other workers who were among seven preparing to enter the tank farm also experienced symptoms.

The four workers with symptoms were taken to Hanford’s occupational medicine provider.

No waste-disturbing work was being done in the AP Tank Farm, and workers were not required to wear respirators there, according to Washington River Protection Solutions.

About two hours later, a worker several miles away at the SY Tank Farm, another group of underground double-shell tanks, also experienced symptoms.

The worker was taken to the Hanford’s occupational medicine provider for evaluation and then transferred to the Richland hospital. A second worker in the SY Tank Farm had no symptoms, but requested an evaluation at Kadlec after going to the on-site medical facility.

Respirators are not required at the SY Tank Farm and information was not immediately available about whether any waste-disturbing work was being done there.

Workers have been reporting symptoms at the tank farms for more than two decades and steps have been taken to reduce exposure through the years.

Those include raising stacks that might vent vapors and increasing monitoring.

In April, Washington River Protection Solutions asked the Savannah River National Laboratory in South Carolina to lead the third independent study of tank vapors in recent years.

A team of the Hanford contractor’s engineers has been assembled to look for potential improvements. It is expected to provide a new assessment of sources for vapor releases, how vapors disperse and equipment options such as taller vent stacks, portable exhausters and filters.

The Chemical Vapors Solutions Team at Hanford, which includes worker and management members, has been reorganized.

Workers also are required to wear respirators in some tank farms or under some conditions when waste is being disturbed, which increases the likelihood that vapors will be released.-- Annette Cary: 509-582-1533; acary@tricityherald.com; Twitter: @HanfordNews